Mosquito symbioses: from basic research to the paratransgenic control of mosquito‐borne diseases

Mosquito‐borne diseases pose significant concerns in public health. Microbial symbionts of mosquitoes are attracting quite a lot of interest in relation to the development of novel strategies aimed to reduce mosquito vectorial capacity with particular regard to paratransgenesis that relies on genetically modified mosquito symbionts to express molecules within the vector able to interfere with parasite development and transmission. Here, we review the present status of the knowledge of mosquito–symbionts relationships, focusing on perspective in the application of symbiotic control in developing an efficient management of mosquito‐borne diseases.     

遗传不育技术在蚊媒疾病防控中的应用

疟疾、登革热等重大传染性蚊媒疾病严重危害人类健康,且目前缺乏有效的药物和疫苗,防治埃及伊蚊、冈比亚按蚊等媒介昆虫是控制和消除这些疾病的有效手段。化学杀虫剂的大规模使用在一定程度上控制了疾病的传播,但其抗药性和环境污染等问题也随之而来。分子生物学的飞速发展为昆虫不育技术(SIT)的更新及害虫防治提供了新的策略,由此发展起来的以释放携带显性致死基因昆虫(RIDL)为代表的一系列遗传不育技术为蚊虫种群防控提供了更加有效的选择。本文概述了遗传技术在蚊虫防控中的应用进展,包括蚊虫遗传防治的历史和策略,阐述了RIDL技术体系的原理,同时介绍了相关遗传控制品系和已经开展的田间释放研究,展示了遗传修饰不育技术在蚊媒疾病防治中的巨大潜力。     

成蚊糖餐行为及其在蚊虫防制中的应用

蚊虫作为重要的病媒生物,其大多数种类在成虫阶段需要取食糖餐,且对不同糖餐组分表现出不同偏好。同时,糖餐行为具有特定的时辰节律。野外条件下,不同成蚊对不同开花植物及其果实具有不同的偏好,不同糖餐植物对成蚊的存活率、寿命、繁殖力的影响各不相同。成蚊对糖餐的定位与植物挥发物有关,目前已有多种糖餐植物的活性挥发物被鉴定。含毒糖诱剂对多种成蚊具有良好的防治效果,人们已将其用于多种病媒生物的防治及蚊媒病毒的检测。利用糖餐植物挥发物研发出蚊虫引诱剂,在降低蚊媒疾病风险方面具有广阔的应用前景。本文对成蚊取食糖餐的习性、成蚊偏好的糖餐植物相关研究以及如何利用成蚊的糖餐行为来进行蚊虫防制等进行了综述。     

Evaluation of a temperate climate mosquito,Ochlerotatus detritus (=Aedes detritus), as a potential vector of Japanese encephalitis virus

The U.K. has not yet experienced a confirmed outbreak of mosquito‐borne virus transmission to people or livestock despite numerous autochthonous epizootic and human outbreaks of mosquito‐borne diseases on the European mainland. Indeed, whether or not British mosquitoes are competent to transmit arboviruses has not been established. Therefore, the competence of a local (temperate) British mosquito species, Ochlerotatus detritus (=Aedes detritus) (Diptera: Culicidae) for transmission of a member of the genus Flavivirus, Japanese encephalitis virus (JEV) as a model for mosquito‐borne virus transmission was assessed. The JEV competence in a laboratory strain of Culex quinquefasciatus (Diptera: Culicidae), a previously incriminated JEV vector, was also evaluated as a positive control. Ochlerotatus detritus adults were reared from field‐collected juvenile stages. In oral infection bioassays, adult females developed disseminated infections and were able to transmit virus as determined by the isolation of virus in saliva secretions. When pooled at 7–21 days post‐infection, 13% and 25% of O. detritus were able to transmit JEV when held at 23 °C and 28 °C, respectively. Similar results were obtained for C. quinquefasciatus. To our knowledge, this study is the first to demonstrate that a British mosquito species, O. detritus, is a potential vector of an exotic flavivirus.     

Current trends in large‐scale viral surveillance methods in mosquitoes

Vector‐borne and zoonotic infectious diseases are serious public health concerns that affect approximately half of the world's population. In particular, arthropod‐borne viruses (arboviruses) have contributed to more mortality and morbidity worldwide with the emergence of dengue, chikungunya, yellow fever, and Zika virus diseases. The infections have scaled up due to urbanization, globalization, and international mobility. Traditionally, the spread of mosquito‐borne viral diseases to humans was considered a low health priority concern. However, their categorization as emerging infectious diseases and public health emergencies of international concern has heightened the attention given by the government, academia, research, and industry for the development of timely, cost‐efficient, and sustainable solutions. The urgency has increased in the wake of global climate change. The focus on effective interventions includes epidemiological monitoring, vector control measures, molecular diagnostics, vaccines, and environmental determinants. In this review, we discuss the etiology and predisposition of mosquito‐borne viruses that are detrimental to public health and economically damaging when disseminated as epidemics. We focus on the large‐scale virus surveillance methods with special reference to innovations and interventions in molecular detection science and technologies that include viral nucleic acid isolation, polymerase chain reaction (PCR)‐based diagnostics, and high‐throughput sequencing technologies. In addition, we discuss the development of a viral RNA extraction and PCR‐based diagnostic kit (Invirustech) that can extract viral RNA from mosquitoes with verified applications in PCR‐based molecular diagnostics of Pan‐flavivirus.     

Genetic engineering of malaria parasite resistance in vector mosquitoes

Malaria is the most significant vector‐borne disease and mostly affects people living in the lesser developed countries of tropical and sub‐tropical regions. Climate changes, rapid global transportation, immigration and invasion of exotic mosquito vectors bring the threat of introduction of the disease to developed nations. Sustainability of malaria control requires the discovery of therapeutic and prophylactic drugs, development of effective vaccines and control of vector mosquitoes. Drug development and vaccine research have been pursued aggressively over the past 20 years, and progress in novel approaches to vector control is now evident. Our long‐term objective is the production and utilization of strains of vector mosquitoes that are genetically refractory to the transmission of malaria parasites. These insects will be used to test the hypothesis that an increase in the frequency of a gene or allele that confers decreased vector competence to a population of mosquitoes will result in a reduction in the incidence and prevalence of malaria. Completed studies make it possible to develop strains of Anopheles mosquitoes expressing specific effector molecules that interfere completely with the transmission of the most lethal human malaria parasite, Plasmodium falciparum. Data are reviewed here that support the use of single‐chain monoclonal antibodies (scFv) that disable parasites in the midgut and hemolymph of transgenic mosquitoes.     

Role of cytochrome P450s in insecticide resistance: impact on the control of mosquito-borne diseases and use of insecticides on Earth

The fight against diseases spread by mosquitoes and other insects has enormous environmental, economic and social consequences. Chemical insecticides remain the first line of defence but the control of diseases, especially malaria and dengue fever, is being increasingly undermined by insecticide resistance. Mosquitoes have a large repertoire of P450s (over 100 genes). By pinpointing the key enzymes associated with insecticide resistance we can begin to develop new tools to aid the implementation of control interventions and reduce their environmental impact on Earth. Recent technological advances are helping us to build a functional profile of the P450 determinants of insecticide metabolic resistance in mosquitoes. Alongside, the cross-responses of mosquito P450s to insecticides and pollutants are also being investigated. Such research will provide the means to produce diagnostic tools for early detection of P450s linked to resistance. It will also enable the design of new insecticides with optimized efficacy in different environments.     

Vector mosquito image classification using novel RIFS feature selection and machine learning models for disease epidemiology

Every year about one million people die due to diseases transmitted by mosquitoes. The infection is transmitted to a person when an infected mosquito stings, injecting the saliva into the human body. The best possible way to prevent a mosquito-borne infection till date is to save the humans from exposure to mosquito bites. This study proposes a Machine Learning (ML) and Deep Learning based system to detect the presence of two critical disease spreading classes of mosquitoes such as the Aedes and Culex. The proposed system will effectively aid in epidemiology to design evidence-based policies and decisions by analyzing the risks and transmission. The study proposes an effective methodology for the classification of mosquitoes using ML and CNN models. The novel RIFS has been introduced which integrates two types of feature selection techniques – the ROI-based image filtering and the wrappers-based FFS technique. Comparative analysis of various ML and deep learning models has been performed to determine the most appropriate model applicable based on their performance metrics as well as computational needs. Results prove that ETC outperformed among the all applied ML model by providing 0.992 accuracy while VVG16 has outperformed other CNN models by giving 0.986 of accuracy.     

Ethical Issues in Field Trials of Genetically Modified Disease‐Resistant Mosquitoes

Mosquito‐borne diseases take a tremendous toll on human populations, especially in developing nations. In the last decade, scientists have developed mosquitoes that have been genetically modified to prevent transmission of mosquito‐borne diseases, and field trials have been conducted. Some mosquitoes have been rendered infertile, some have been equipped with a vaccine they transmit to humans, and some have been designed to resist diseases. This article focuses on ethical issues raised by field trials of disease‐resistant, genetically modified mosquitoes. Some of these issues include: protecting the public and the environment from harm, balancing benefits and risks, collaborating with the local community, avoiding exploitation, and safeguarding the rights and welfare of research subjects. One of the most difficult problems involves protecting the welfare of community members who will be impacted by the release of mosquitoes but who are not enrolled in the study as research subjects. To address this concern, field trials should take place only when the targeted disease is a significant public health problem in an isolated area, the benefits of the trial for the community are likely to outweigh the risks, community leaders approve of the trial, and there are measures in place to protect the welfare of un‐enrolled community members, such as informing the community about the study and offering free treatment to people who contract mosquito‐borne diseases. Since the justification of any field trial depends on a careful examination of the scientific and ethical issues, proposed studies should be evaluated on a case‐by‐case basis.     

Identification of gravid mosquitoes from changes in spectral and polarimetric backscatter cross sections

Improving the survey of mosquito populations is of the utmost importance to further enhance mitigation techniques that protect human populations from mosquito‐borne diseases. While mosquito populations are generally studied using physical traps, stand‐off optical sensors allow to study insect ecosystems with potentially better spatial and temporal resolution. This can be greatly beneficial to eco‐epidemiological models and various mosquito control programs. In this contribution, we demonstrate that the gravidity of female mosquitoes can be identified from changes in their spectral and polarimetric backscatter cross sections. Among other predictive variables, the wing beat frequency and the depolarization ratio of the mosquito body allows for the identification of gravid females with a precision and recall of 86% and 87%, respectively. Since female mosquitoes need a blood meal to become gravid, statistics on gravidity is of prime importance as only females that have been gravid might carry infectious diseases. In addition, it allows to detect possible breeding habitat, predict a potential increase in the mosquito population and provide a better overall understanding of the ecosystem dynamics. As a result, targeted and localized mitigation techniques can be used, reducing the cost and improving the efficiency of mosquito population control.     

Impact of environment on mosquito response to pyrethroid insecticides: Facts,evidences and prospects

By transmitting major human diseases such as malaria, dengue fever and filariasis, mosquito species represent a serious threat worldwide in terms of public health, and pose a significant economic burden for the African continent and developing tropical regions. Most vector control programmes aiming at controlling life-threatening mosquitoes rely on the use of chemical insecticides, mainly belonging to the pyrethroid class. However, resistance of mosquito populations to pyrethroids is increasing at a dramatic rate, threatening the efficacy of control programmes throughout insecticide-treated areas, where mosquito-borne diseases are still prevalent. In the absence of new insecticides and efficient alternative vector control methods, resistance management strategies are therefore critical, but these require a deep understanding of adaptive mechanisms underlying resistance. Although insecticide resistance mechanisms are intensively studied in mosquitoes, such adaptation is often considered as the unique result of the selection pressure caused by insecticides used for vector control. Indeed, additional environmental parameters, such as insecticides/pesticides usage in agriculture, the presence of anthropogenic or natural xenobiotics, and biotic interactions between vectors and other organisms, may affect both the overall mosquito responses to pyrethroids and the selection of resistance mechanisms. In this context, the present work aims at updating current knowledge on pyrethroid resistance mechanisms in mosquitoes and compiling available data, often from different research fields, on the impact of the environment on mosquito response to pyrethroids. Key environmental factors, such as the presence of urban or agricultural pollutants and biotic interactions between mosquitoes and their microbiome are discussed, and research perspectives to fill in knowledge gaps are suggested.     

Toxicity of plants as insecticides against human pathogenic mosquito vectors of Saudi Arabian strains—A review

Mosquitoes (Diptera: Culicidae) are considered to be huge threat among millions of peoples, animals, and other living organisms in the world. Most of the vector borne diseases such as malaria, filariasis, dengue hemorrhagic fever, chikungunya, Japanese encephalitis etc., created huge impact on humans in all over the world. Vector diseases in Kingdom of Saudi Arabia are increasing day by day and their control measures taken through the government sectors for eradicating the vectors helps in controlling the diseases but still more approaches to be implemented or assimilated. Most of the synthetic or chemical based insecticides to control mosquitoes developed resistance among their communities even though they showed their potential in controlling the vector in initial days. Botanical insecticides from plant-based origin such as active compounds, essential oils, green synthesized nanomaterials, and microbial secondary metabolites helps more efficiency in controlling vectors. Mode of action against vectors differs based on its persisting active ingredients, such as larvicidal, pupicidal, adulticidal, oviposition, morphological changes etc. Even though number of research works has been carried out against mosquito species, there is only limited number of studies undergone against mosquito vectors from Saudi Arabia origin. Hence this review will give us the current knowledge on the effectiveness of botanical insecticides against major mosquito vectors from Saudi Arabia. Thus, it gives more significant against medical and veterinary sectors.     

Laboratory and field testing of bednet traps for mosquito (Diptera: Culicidae) sampling in West Java,Indonesia

Surveillance of medically important mosquitoes is critical to determine the risk of mosquito‐borne disease transmission. The purpose of this research was to test self‐supporting, exposure‐free bednet traps to survey mosquitoes. In the laboratory we tested human‐baited and unbaited CDC light trap/cot bednet (CDCBN) combinations against three types of traps: the Mbita Trap (MIBITA), a Tent Trap (TENT), and a modified Townes style Malaise trap (TSM). In the laboratory, 16 runs comparing MBITA, TSM, and TENT to the CDCBN were conducted for a total of 48 runs of the experiment using 13,600 mosquitoes. The TENT trap collected significantly more mosquitoes than the CDCBN. The CDCBN collected significantly more than the MBITA and there was no difference between the TSM and the CDCBN. Two field trials were conducted in Cibuntu, Sukabumi, West Java, Indonesia. The first test compared human‐baited and unbaited CDCBN, TENT, and TSM traps during six nights over two consecutive weeks per month from January, 2007 to September, 2007 for a total of 54 trapnights. A total of 8,474 mosquitoes representing 33 species were collected using the six trapping methods. The TENT‐baited trap collected significantly more mosquitoes than both the CDCBN and the TSM. The second field trial was a comparison of the baited and unbaited TENT and CDCBN traps and Human Landing Collections (HLCs). The trial was carried out from January, 2008 to May, 2008 for a total of 30 trap nights. A total of 11,923 mosquitoes were collected representing 24 species. Human Landing Collections captured significantly more mosquitoes than either the TENT or the CDCBN. The baited and unbaited TENT collected significantly more mosquitoes than the CDCBN. The TENT trap was found to be an effective, light‐weight substitute for the CDC light‐trap, bednet combination in the field and should be considered for use in surveys of mosquito‐borne diseases such as malaria, arboviruses, and filariasis.     

Attraction of mosquitoes to primate odours and implications for zoonotic Plasmodium transmission

Vector-borne diseases often originate from wildlife and can spill over into the human population. One of the most important determinants of vector-borne disease transmission is the host preference of mosquitoes. Mosquitoes with a specialised host preference are guided by body odours to find their hosts in addition to carbon dioxide. Little is known about the role of mosquito host preference in the spillover of pathogenic agents from humans towards animals and vice versa. In the Republic of Congo, the attraction of mosquitoes to primate host odours was determined, as well as their possible role as malaria vectors, using odour-baited traps mimicking the potential hosts of mosquitoes. Most of the mosquito species caught showed a generalistic host preference. Anopheles obscurus was the most abundant Anopheles mosquito, with a generalistic host preference observed from the olfactory response and the detection of various Plasmodium parasites. Interestingly, Culex decens showed a much higher attraction towards chimpanzee odours than to human or cow odours. Human Plasmodium parasites were observed in both human and chimpanzee blood, although not in the Anopheles mosquitoes that were collected. Understanding the role of mosquito host preference for cross-species parasite transmission provides information that will help to determine the risk of spillover of vector-borne diseases.     

Effects of a larval mosquito biopesticide and Culex larvae on a freshwater nanophytoplankton (Selenastrum capricornatum) under axenic conditions

The effects of microbial biopesticides used for mosquito control on autotrophic microorganisms such as nanophytoplankton are equivocal. We examined impacts of mosquito biopesticides and mosquito larvae on primary producers in two independent experiments. In the first experiment, we examined the effects of a commonly used microbial biopesticide formulation (VectoMax^® CG) on a unicellular microalga, Selenastrum capricornatum Printz, under axenic laboratory conditions. The biopesticide treatments included two concentrations (0.008 and 0.016 g liter^?1) of VectoMax^® CG and two controls (one untreated and another with autoclaved 0.016 g VectoMax^® CG liter^?1) in replicated axenic experimental microcosms. Spectrophotometric analysis of chlorophyll a (proxy for algal biomass) and direct enumeration of algal cells following the treatments revealed no significant effects of the microbial biopesticide on algal population growth during the four‐week study. In the second experiment, we tested the effects of different densities of Culex larvae on the population of S. capricornatum. Effects of mosquito larvae feeding on S. capricornatum were significant with a curvilinear relationship between larval density and algal abundance in the water column. Together, these studies demonstrated a lack of direct cytological/toxicological effects of Bacillus‐based microbial pesticides on freshwater primary production and support the hypothesis that the reduction in algal primary production previously reported when Bti products were applied to aquatic environments was likely independent of the Bacillus‐based larvicidal toxins. Instead, it was likely mediated by microbial interactions in the water column and the trophic cascade effects that resulted from the removal of larval mosquitoes. These studies suggest that mosquito larvae independent of pesticide application can influence primary production. Our method of evaluating biopesticides against small photoautotrophs can be very useful for studying the unintended effects on autotrophic microorganisms of other pesticides, including herbicides and pesticides applied to aquatic environments.     

蚊虫综合防治进展

近年来,蚊媒病在世界许多地区再度猖獗。例如有些国家的疟疾发病有增无减,东南亚的登革出血热(dengue haemorrhagic fever)扩展到了加勒比海区。因而蚊虫依然是医学昆虫防治的重点。自从本刊《蚊虫综合防治》一文(陆宝麟,1978)刊登以来,在短短的几年中,在防治方针策略方面有了明显的变更,在防治手段方法方面有了较大的提高。本文乃继前文之后,对这方面的重要进展择要作一综合介绍。     

Innate preference or opportunism: mosquitoes feeding on birds of prey at the Southeastern Raptor Center

The amplification of mosquito‐borne pathogens is driven by patterns of host use by vectors. While each mosquito species is innately adapted to feed upon a particular group of hosts, this “preference” is difficult to assess in field‐based studies, because factors such as host defenses and spatial and temporal overlap of mosquitoes and hosts affect which host animals actually get bitten. Here we examined patterns of host use by mosquitoes feeding on caged raptors at a rehabilitation and education center for birds of prey in Alabama, U.S.A. PCR‐based techniques were used to determine the host species fed upon. Of 19 raptor species at the facility, seven were found to be fed upon by mosquitoes. Feeding indices and linear regression indicated that no species or family of raptor were significantly preferred over another (R²=0.46). Relative abundance adjusted for bird size explained a statistically significant amount of the variation in relative host use (R²=0.71), suggesting that bird size is an important component of host selection by mosquitoes. These findings support the hypothesis that traits of host animals drive patterns of host use by mosquitoes in nature, an interaction that leads to amplification of mosquito‐borne viruses.     

The role of vector control in stopping the transmission of malaria: threats and opportunities

Malaria control, and that of other insect borne diseases such as dengue, is heavily dependent on our ability to control the mosquito populations that transmit these diseases. The major push over the last decade to reduce the global burden of malaria has been driven by the distribution of pyrethroid insecticide-treated bednets and an increase in coverage of indoor residual spraying (IRS). This has reduced malaria deaths by a third. Progress towards the goal of reducing this further is threatened by lack of funding and the selection of drug and insecticide resistance. When malaria control was initially scaled up, there was little pyrethroid resistance in the major vectors, today there is no country in Africa where the vectors remain fully susceptible to pyrethroids. The first pyrethroid resistance mechanisms to be selected produced low-level resistance which had little or no operational significance. More recently, metabolically based resistance has been selected, primarily in West Africa, which in some mosquito populations produces more than 1000-fold resistance. As this spreads the effectiveness of pyrethroid-based bednets and IRS will be compromised. New public health insecticides are not readily available. The pipeline of agrochemical insecticides that can be re-purposed for public health dried up 30 years ago when the target product profile for agricultural insecticides shifted from broad spectrum, stable, contact-acting insecticides to narrow spectrum stomach poisons that could be delivered through the plant. A public–private partnership, the Innovative Vector Control Consortium, was established in 2005 to stimulate the development of new public health pesticides. Nine potential new classes of chemistry are in the pipeline, with the intention of developing three into new insecticides. While this has been successfully achieved, it will still take 6–9 years for new insecticides to reach the market. Careful management of the resistance situation in the interim will be needed if current gains in malaria control are not to be reversed.     

Activation of Akt Signaling Reduces the Prevalence and Intensity of Malaria Parasite Infection and Lifespan in Anopheles stephensi Mosquitoes

Malaria (Plasmodium spp.) kills nearly one million people annually and this number will likely increase as drug and insecticide resistance reduces the effectiveness of current control strategies. The most important human malaria parasite, Plasmodium falciparum, undergoes a complex developmental cycle in the mosquito that takes approximately two weeks and begins with the invasion of the mosquito midgut. Here, we demonstrate that increased Akt signaling in the mosquito midgut disrupts parasite development and concurrently reduces the duration that mosquitoes are infective to humans. Specifically, we found that increased Akt signaling in the midgut of heterozygous Anopheles stephensi reduced the number of infected mosquitoes by 60–99%. Of those mosquitoes that were infected, we observed a 75–99% reduction in parasite load. In homozygous mosquitoes with increased Akt signaling parasite infection was completely blocked. The increase in midgut-specific Akt signaling also led to an 18–20% reduction in the average mosquito lifespan. Thus, activation of Akt signaling reduced the number of infected mosquitoes, the number of malaria parasites per infected mosquito, and the duration of mosquito infectivity.